Exactly what caused the drop is unclear, but the researchers noted that trans fats are often found in manufactured foods, and in 2003, the Food and Drug Administration began to require that food products list their trans fat content on their nutrition labels.

In addition, around that time, manufacturers started to remove trans fats from their foods, and there was an increase in educational information about the health risks of these fats, said study researcher Hubert Vesper, of the Centers for Disease Control and Prevention. All of these factors could have contributed to the decline.

In the study, the researchers examined blood samples from 229 adults taken in 2000 as part of a national health survey, and 292 samples taken in 2009. The blood samples were analyzed for the presence of four major trans fatty acids. Trans fatty acids are stable in the blood, and so would not have degraded between the time the samples were taken and analyzed, Vesper said.

The study did not include other ethnic groups, so the findings cannot be generalized to the population as a whole, the researchers said. Further research is needed to see whether a drop in cholesterol levels occurred along with the drop in trans fat levels.

The researchers plan to further analyze their data to better assess the effectiveness of public health measures in reducing trans fat levels, Vesper said.

The study is published in the Feb. 8 issue of the Journal of the American Medical Association.

Pass it on: White Americans experienced a decline in their blood levels of trans fat, which could mean a decreased risk of heart disease for this group.

Rachael Rettner

Rachael has been with Live Science since 2010. She has a masters degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a Bachelor of Science in molecular biology and a Master of Science in biology from the University of California, San Diego.